The questions 'what is healthy aging?'and 'how do we promote it'are increasingly relevant due to our rapidly expanding elderly population. Aging is a complex systemic phenomenon that results from concomitant deterioration of multiple physiological systems resulting in diminished physical and cognitive function and decreased ability to adapt to stress. Our research team and others have shown that the integrity and interdependence of aging physiologic systems can be studied by examining the "complexity" of their moment-to-moment dynamics using quantitative tools drawn from systems biology and complexity theory. These studies suggest that: 1) aging is associated with loss of fractal-like complexity in multiple physiological systems;and 2) indices of complexity are often more sensitive predictors of health and disease than conventional measures. However, little research has evaluated whether the physiological complexity lost with age can be attenuated or restored with targeted interventions. Tai Chi is a multi-component mind-body exercise grounded in the holistic model of traditional Chinese medicine. Its explicit goals of targeting multiple physiological processes and integrating their dynamics make it particularly suited for evaluation within a complexity framework. Tai Chi is reported to improve many age- related health issues such as cardiovascular function, balance, gait, self-efficacy, and quality of life. To date, few have attempted to study Tai Chi's impact on age-related physiological processes using complexity measures and to relate such measures to functional status. Such knowledge could generate improved clinical and mechanistic understanding of how multi-component mind-body exercises impact health and aging. To evaluate Tai Chi's effect on age-related loss of complexity, we propose a two-arm prospective randomized clinical trial. Our overarching goal is to evaluate if six months of Tai Chi training, compared to a waitlist control receiving standard medical care, can enhance physiological complexity and adaptability in older Tai Chi-naive adults. Secondary goals of the study are to characterize the relationship between complexity biomarkers, measures of function, and resilience. This pilot study will inform a future more definitive trial by providing information on recruitment and retention, compliance, dose-dependent effects, preliminary estimates of effect size, and the optimal biomarkers of complexity, function, and adaptive capacity. PUBLIC HEALTH RELEVANCE: Understanding the biology of aging, and evaluating interventions that promote healthy aging are becoming increasingly important priorities. Using mathematical tools drawn from the emerging field of complexity and systems biology, this study will characterize how Tai Chi--a multipcomponent mind-body exercise--impacts moment-to-moment variations in heart beat, gait, and balance control, as well as the relationship of these patterns to more traditional measures of physical and mental function. Results of this study may lead to novel measures that help us monitor and understand the physiological processes of aging, and explore the potential benefits of Tai Chi for healthy aging.